The present invention relates to an improved seismic isolation structure utilizing a toggle linkage.
By way of background, there are in use two common types of seismic isolation devices utilizing viscous dampers. One type is a diagonal brace structure incorporating a viscous damper, which is placed in a frame of a structure, such as a building. Another type of device is a chevron structure which is placed in the frame of a building. The seismic displacement which is opposed by the foregoing seismic isolation devices is the horizontal displacement between the floors of a building or between various levels of other structures, such as bridges, and it is this displacement which must be used to drive the viscous damper. However, in diagonal and chevron isolation devices, the damper has a very small displacement as the various levels of a structure move relative to each other, thereby requiring large, heavy, short stroke dampers which are relatively expensive both in initial cost of fabrication and cost of installation. By way of broad example, the relative movement between floors of a building could be on the order of a fraction of an inch. Thus, for example, in a rectangular frame of a building wall having a dimension of about 22 feet horizontally and 20 feet vertically and having a diagonal of about 30 feet, the change in length of the diagonal would be only a fraction of an inch. This small fraction of an inch in change in length of a diagonal constitutes the stroke which has to be applied to the viscous damper, thereby necessitating the above-mentioned relatively large, heavy, short stroke dampers. Another type of seismic isolation device which is known in the prior art is a toggle linkage such as shown in opened Japanese patent application Sho 63-114069 (Patent No. 1-284639). The advantage of a toggle linkage is that it essentially magnifies relatively small movements between levels of a structure, that is, it provides a motion which is larger than the motion produced by the change in length of a diagonal brace or by the movement of a chevron brace. Thus, a toggle brace permits the use of relatively inexpensive long stroke, relatively light hydraulic dampers and also permits the use of other types of long stroke shock absorbers. However, toggle linkages with clevis types of connections at the junctions of the links of a toggle linkage have certain deficiencies, namely, (1) there is too much play at the clevis so that the shifting of the floors of a building is not fully transmitted by the links of the toggle linkage to the damper, and (2) the clevis connection inherently permits out-of-plane buckling which further diminishes the amount of floor shifting which is effectively transmitted to the toggle linkage.
In copending application Ser. No. 08/694,153, filed Aug. 8, 1996, a solid joint in the form of a metal plate replaced a clevis joint between the inner ends of a toggle linkage to eliminate play at the clevis and also eliminate out-of-plane buckling of the toggle linkage. However, it was discovered that, in cases where the metal plate was made relatively thin, the metal plate did not bend accurately along a desired pivot axis between the two links which it connected. More specifically, instead of bending about a desired well defined axis, it bent in a complex undulating shape which, in turn, caused the stroke of the damper to be shortened. It is believed that this was due to the fact that the link which contained the damper was secured to one of the toggle links, and thus there was an unequal force distribution on the links which were connected to each other by the metal plate due to the fact that the force of the damper was applied onto only one of the links when a seismic shock was experienced. It is with overcoming the foregoing deficiency of a toggle linkage having a prior type of plate connection that the present invention is concerned.